1. Field of the Invention
This invention relates generally to an MEA for a fuel cell that includes a multi-layer catalyst configuration for decreasing the hydrogen and/or oxygen partial pressure at the cell membrane and, more particularly, to an MEA for a fuel cell that includes multiple catalyst layers on the anode side and/or cathode side of the cell membrane that reduces the hydrogen and oxygen partial pressure at the membrane so as to decrease membrane degradation.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane defines a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Hydrogen permeates through the membrane from the anode side to the cathode side and oxygen permeates through the membrane from the cathode side to the anode side of the fuel cells in the stack, often referred to as hydrogen cross-over and oxygen cross-over, respectively. The simultaneous presence of hydrogen, oxygen and catalyst (typically Pt or Pt/transition-metal alloys) leads to chemical degradation of the ion-conducting polymer in the membrane and in the electrodes. One of the proposed mechanisms to explain this phenomenon is that hydrogen and oxygen can react to generate hydrogen peroxide. The hydrogen peroxide reacts vigorously with ferric and/or ferrous ions present as impurities in the fuel cell components or are generated by corrosion of the bipolar plates. The reaction of hydrogen peroxide and ferrous ions produces hydroxyl free radicals, which degrade the membrane as a result of Fenton's reaction shown below.H2O2+Fe2+→Fe3++HO−+HO*  (1)RH+OH*→R*+H2O  (2)R*+Fe3+→R++Fe2+  (3)
The degradation of the membrane produces primarily hydrogen fluoride in the fuel cell, referred to herein as a fluoride release rate from the membrane. The hydrogen fluoride corrodes stainless steel bipolar plates, which generates more ferric and ferrous ions, increasing the production of the hydroxyl free radicals, and thus further increasing the degradation of the membrane. This process thus becomes auto-catalytic and a significant degradation of both the membrane and the bipolar plate occurs as a result of the reaction.
It has been discovered that decreasing the partial pressure of hydrogen on the anode side of the fuel cell and/or decreasing the oxygen partial pressure on the cathode side of the fuel cell decreases the fluoride release rate.